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Advise on a torsion box benchtop

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Forum topic by localpref posted 07-23-2012 at 01:46 PM 1971 views 1 time favorited 14 replies Add to Favorites Watch
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localpref

5 posts in 779 days


07-23-2012 at 01:46 PM

I built a workbench/assembly table top I used mdf to build the box then wrapped the edges in oak and put a hard board on the top. When I was gluing it up in an attempt to insure good pressure I put to much weight in the middle and ended up with a sag in the middle. It is roughly 6 foot long by 3 feet wide. It looks like the sag is about an 8th of an inch I am wondering is there an simple way to correct this or will in need to build a new one? I’ll attach a picture of the top as well. Any advice would be appreciated I really don’t want to have to cut all those MDF ribs again.


14 replies so far

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levan

405 posts in 1616 days


#1 posted 07-23-2012 at 02:08 PM

Sounds like you need to rout it flat, by taking 1/8 ” off the high points. There are lots of planing with a router, jigs on here I’m sure you can you find one that will work. Perhaps you could use contact cement on the new hard board. or simply just nail on 1/4” masonite once you have it flat.
best wishes

-- Lynn "If you think you can do a thing or think you can't do a thing, you're right". Henry Ford

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crank49

3380 posts in 1608 days


#2 posted 07-23-2012 at 03:16 PM

I think you will have to take the skins off and re-surface it.
I wouldn’t ruin a good router bit planing hardboard.

This is the problem with any builtup workbench top.
If you glue it together with anything other than a perfect flat surface, you just have to re-do it.

Mine was built of 4 layers of 3/4” plywood and had a little twist in the first two layers. Lucky I caught it before the glue set and was able to get it apart. Cleaned it up and put it together again.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

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localpref

5 posts in 779 days


#3 posted 07-23-2012 at 06:38 PM

I did not glue the hard board down just used screws so that is a bit of luck. I’ll run a search on planning jigs anyone have any good links or advice?

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waho6o9

4850 posts in 1214 days


#4 posted 07-23-2012 at 07:39 PM

that happened to mine as well localpref. I just junked it.

Good luck on yours though, keep us posted.

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newwoodbutcher

347 posts in 1487 days


#5 posted 07-23-2012 at 07:41 PM

I would start over

-- Ken

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localpref

5 posts in 779 days


#6 posted 07-24-2012 at 07:32 AM

I did some searching on using a router with a sled jig to flatten out a tabletop. Looks like getting the jig setup up for the very tight tolerance might be a bit of a challange also anyone know how the MDF would react to this type of tooling?

wondering if I am just going to have to bite the bullet and rebuild. Any suggestions on how to insure I get it assembeled dead flat if I go this route?

View Mainiac Matt 's profile

Mainiac Matt

3905 posts in 965 days


#7 posted 07-24-2012 at 08:10 AM

Any suggestions on how to insure I get it assembeled dead flat if I go this route?

I’m not sure that I have a clear picture of your build up…. do you have bracing within the torsion box perimeter? if so what is the spacing?

I build a mobile stand for a bench top mill using torsion boxes, and the thing is a total beast….. so I’ll offer a few thoughts…

you don’t need a lot of clamp pressure… just enought to squeeze out the glue…. and if you apply plenty of glue, that won’t take much.

a couple brads or finish nails go a long way towards keeping things lined up and if you tack around the perimeter, you’ll be less likely to sag the middle, as the panel will be held taunt.

only apply clamp pressure over a totally solid area. if you apply clamp pressure over one of the voids, you will bow your panel and when the glue is dried and the clamp pressure released released, the bow will be locked in place.

cross bracing every ~12” or so (with stock ripped to the same thickness as the perimeter) will ad a lot of strength and rigidity.

I don’t think you’re going to be able to fix this…. either live with it… or use it for something else (hack bench) and make another for your “dead flat” top.

good luck

-- Pine is fine, but Oak's no joke!

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PurpLev

8476 posts in 2285 days


#8 posted 07-24-2012 at 08:34 AM

the idea of a torsion box is to build it flat to begin with, if that part is messed up… the rest will follow suit.

AFAIR Marc (Thewoodwhisperer) had a good video some years ago about building torsion box, and he put attention to the setup used to building it to ensure that it will come out flat (leveling the sawhorses it is build upon and using flat stretchers and all).

routing a 6’ x 3’ MDF is going to be real hard on YOU and on your router bits… MDF is cheap enough that I would take it apart and redo it, or just do it from scratch again.

dont even wannt get into all that FINE DUST that will come out of routing MDF which is the worst dust for your lungs…

-- ㊍ When in doubt - There is no doubt - Go the safer route.

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waho6o9

4850 posts in 1214 days


#9 posted 07-24-2012 at 09:10 AM

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crank49

3380 posts in 1608 days


#10 posted 07-24-2012 at 10:06 AM

Mine was not torsion box, it was 4 layers of 3/4 plywood, but the same principal applies.
Build it anything other than flat true and square and it will need to be rebuilt.
I know because I started out without paying enough attention to this and had to start over.

Second time i did the following:
I setup two saw horses and leveled each and then leveled to each other. This is critical.

Next I set two 8 ft box-beams, made of dimensional lumber, across the two saw horses.
The beams were built on the flattest place I could find and I took care to be sure they were straight by sighting along their length; often. They were glued and screwed 1×6s and 1×4s

I used winding sticks to be sure the beams were flat and parallel and not twisted.
Double checked for level in all directions

This is the base upon which I builit my top.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

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rockindavan

283 posts in 1273 days


#11 posted 07-24-2012 at 10:15 AM

Just a thought, what about using epoxy as a leveler, then screw the hardboard to the top, or just leave the epoxy coating as your surface

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localpref

5 posts in 779 days


#12 posted 07-24-2012 at 10:47 AM

ssnvet
Yes there is bracing inside I built it quit awhile ago but I think the spines are about an inch and half thick they have a dado every 6 inches that is as wide as the mdf and half the height of the spines. This way the ones that go lengthwise intersect with the ones that go across and form a grid. I hope I explained that okay. Really wish I had a picture of it prior to gluing it all together. The problem came when I glued it up I put some really heavy stuff in the middle which caused it to sag while the glue was drying.

Rockindaven
Have you ever used this method to level up a table. Since it is going to be used as a work bench I would be afraid it might crack when I am banging stuff on top of it. Any idea what type I should use and about how much the cost would be for a 6 by 3 foot table?

Crank,
I think you are right I should have thought out the surface I was using to support the table while gluing up.

All
After work today I am going to start devising a plan to construct a new top. I’ll post here when I am done to get a sanity check and some pointers before I do anything. I’ll try and post the picture again as it did not seem to take in the 1st post. Also does this seem like a good benchtop for someone looking to start woodworking that will pirmarly use power tools?

View brtech's profile

brtech

665 posts in 1559 days


#13 posted 07-24-2012 at 02:38 PM

Is this an assembly table or a workbench? Mostly, do you need workholding on the surface? The problem with a torsion box top on a workbench is that it’s hard to put dog holes in it to hold work down on the table surface. You can always manage to get a vise on an edge, but not on the surface.

A router sled for this application would be pretty easy, and useful for projects in the future. I’d joint a couple of 2×4s and get them level on the side of the work bench and use a big wide cutter on the router. Sure, lots of dust, but it would be pretty easy and pretty fast to smooth it out. Take the hardboard out and do it on the MDF, then put the hardboard back on.

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rockindavan

283 posts in 1273 days


#14 posted 07-24-2012 at 10:50 PM

Can’t say for sure if the epoxy would crack or not, but from my experience it is spectacular and has surpassed my expectations every time I have used it. Problem is, that it costs about $170 for a gallon with hardener and pump from West Systems. You could buy the smaller size container instead if its a little too steep, and it would still be enough. If you buy it it will last you many many years and it brings a whole new appreciation for what epoxy can do. I figured I would throw out the suggestion, but if money is a concern, the investment might not be worth it.

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